import: enable sparse file writing logic only for files we create

import: enable sparse file writing logic only for files we create

Only if we create a file we know for sure that it is empty and hence our
sparse file logic of skipping over NUL bytes can can work. If we hwoever
are called to write data to some existing file/block device, we must do
regular writes to override everything that might be in place before.

Hence, conditionalize sparse file writing on the write offset not being
configured (which is how we internally distinguish write to existing
file and write to new file)